Mechanism for giant Cl− and F− enhancements in electron-induced dissociation of CF2Cl2 coadsorbed with water or ammonia ices

We have observed giant Cl− and F− enhancements by factors of 102–104 in electron-induced dissociation of a fractional monolayer of CF2Cl2 when CF2Cl2 is coadsorbed with water ice or ammonia ice on a surface. The enhancement of Cl− is much stronger than that of F−, and the enhancements for both ion species by NH3 coadsorbate are stronger than those by H2O. Moreover, the magnitudes of enhancement increase strongly with decreasing CF2Cl2 concentration. Only small negative-ion enhancements are observed for coadsorption with nonpolar CH4 molecules. It is proposed that giant negative-ion enhancements are due to dissociation of CF2Cl2 by capture of electrons self-trapped in polar water or ammonia molecules. This process may have far-reaching implications for the ozone-hole creation in the polar stratosphere.